基于Semaphore实现Limiter限流器

简介

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Semaphore 信号量是用来限制多个线程访问同一个资源，简单来说就是只能有特定数量的线程同时访问资源。与锁不同的是，锁只允许一个线程访问特定的资源，而信号量是允许指定数量的线程同时访问，可以用来协调多线程。

Semaphore 并不是Java语言所特有的，几乎所有的并发语言都有。信号量模型都是一样的，如下：

由计数器、队列和三个方法所组成。

计数器 ：记录还可以有多少个线程来访问资源
等待队列 ：将等待资源的线程放入此队列

/**
     * Acquires a permit from this semaphore, blocking until one is
     * available, or the thread is {@linkplain Thread#interrupt interrupted}.
     * 阻塞到获取许可
     * 
Acquires a permit, if one is available and returns immediately,
     * reducing the number of available permits by one.
     * 获取许可，如果有一个许可是可用的，立即返回并减少数量
     * 
If no permit is available then the current thread becomes
     * disabled for thread scheduling purposes and lies dormant until
     * one of two things happens:
     * 

     * 
Some other thread invokes the {@link #release} method for this
     * semaphore and the current thread is next to be assigned a permit; or
     * 
Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread.
     * 
     * 如果没有许可可用，那么当前线程就会不可线程调度并且休眠，直到：
     * 1、其他线程调用了release方法释放了该信号量的许可证，并且当前线程是下一个被分配到许可证的
     * 2、其他线程中断了当前线程
     * 
If the current thread:
     * 

     * 
has its interrupted status set on entry to this method; or
     * 
is {@linkplain Thread#interrupt interrupted} while waiting
     * for a permit,
     * 
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * @throws InterruptedException if the current thread is interrupted
     */
    public void acquire() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

Limiter

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基于 Semaphore 可以很轻松的实现限流，比如同一时间内只能有 5 个线程执行：

private static final int THREAD_COUNT = 100;
    private static ExecutorService executorService = Executors.newFixedThreadPool(THREAD_COUNT);
    private static Semaphore semp = new Semaphore(5);
    public static void main(String[] args) {
        for (int i = 0; i < THREAD_COUNT; i++) {
            executorService.execute(()->{
                try {
                	// 如果无法获取许可，将会一直阻塞...
                    semp.acquire();
                    System.out.println("Thread - " + Thread.currentThread().getName() +" sleep ...");
                    Thread.sleep(1000);
                    semp.release();
                } catch (InterruptedException e) {
                	// 如果被中断，则会抛出异常
                    e.printStackTrace();
                }finally {

                }
            });
        }
    }

也可以基于 Semaphore 对特定的特定的资源进行限流。

public class Limiter {


    private Semaphore semaphore = new Semaphore(50);

    private Lock lock = new ReentrantLock();

    public boolean acquire() {
        //lock.lock();
        try {
            // acquire会阻塞
            semaphore.acquire();
            return true;
        } catch (InterruptedException e) {
            // 当前线程被中断时会抛出 InterruptedException
            // Ingore it
            //e.printStackTrace();
            return false;
        } finally {
            //lock.unlock();
        }
    }

    public void release() {
        semaphore.release();
    }

    public static void main(String[] args) {
        Limiter limiter = new Limiter();
        for (int i = 0; i < 10000; i++) {
            new Thread(() -> {
                try {
                    if (limiter.acquire()) {
                        System.out.println("sleep------>" + Thread.currentThread().getName());
                        Thread.sleep(10000);
                        System.out.println("wake------>" + Thread.currentThread().getName());
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    limiter.release();
                }
            }, "Thread-" + i).start();

        }
    }

}